Heated Body. A creature that touches the remorhaz or hits it with a melee attack while within 5 feet of it takes 10 (3d6) fire damage.

I believe that with this wording, if you are swallowed you are definitely touching the creature, so the fire damage would apply.

What do you think does this monster weight?

I speak here as my table's resident rules lawyer. Heated Body is one of the most abysmally written game features I have ever seen. The trait activates whenever the remorhaz is hit with a melee attack by a creature within 5 feet. That much is fine. A melee attack is a discrete event that we can pin down for the purpose of implementing the trait in battle. But this wording gives no indication for what is meant by touching the remorhaz. It is possible (and rather frequent) for a creature to be touching the remorhaz for more than one instant, such as when grappled or swallowed by the monster. This trait gives no indication for how often the damage occurs in this event, whether it is at the start of a given creature's turn, or even whether it can happen multiple times in a turn. So I'm going to calculate this monster's challenge rating to see what makes the most sense for a party fighting a monster with this challenge rating.

The monster has 195 hit points and 17 AC, so its defensive challenge rating is 9.

The monster deals 40+10 damage with its bite, and can be assumed to deal 21 damage twice with its stomach. 40+10+(21*2/3)=64. Its attack bonus is +11. If we ignore its Heated Body, the monster's offensive challenge rating is 12.

Even without Heated Body, the monster's challenge rating is already 10.5 rounded up to 11. That's troubling. If we assume that there are even two melee characters hitting this thing from within 5 feet twice a turn, thus incurring the trait four times per round, then this monster's challenge rating skyrockets faster than GameStop's stock last January. Fortunately, the Dungeon Master's Guide has something to say about this trait. In Chapter 9, in the bit where they talk about calculating challenge ratings, they mention that an azer's Elemental Body effectively increases the monster's per-round damage by the amount noted in the trait. But if you look at the azer's stat block, you'll see that the trait is actually called Heated Body and has identical wording to the remorhaz's Heated Body.

The monster is challenge rating 2, and is expected to be used against low level adventurers without Extra Attack. Even if a party relies on two melee fighters, at least one of them probably has a reach weapon, nullifying the trait, so it makes sense for an azer to deal damage with this trait only once per round. A remorhaz's Heated Body, however, is not so easily circumvented, since its challenge rating is so high that a single applicable melee fighter will take the damage twice. It's possible that the remorhaz touching and swallowing adventurers counts as the adventurers touching the remorhaz and can thus activate Heated Body, but let's see if Heated Body is enough without factoring that in.

So Heated Body effectively adds 20 to the damage output (thanks to a melee fighter's Extra Attack), increasing it to 84. With its attack bonus of +11, the offensive challenge rating is now 14. The final challenge rating is 11.5 rounded down to 11. So it seems like the best way to rule this trait is to make sure that the remorhaz's touches and attacks (as opposed to the opponents' touches and attacks) do not trigger Heated Body.

Quote from LeSweetLoaf >>

just throw your bear totem barbarian at it, half damage to everything

That actually happened to us last night. Our DM sicced a remorhaz on us, and the bear totem barbarian DMPC got swallowed. The poor remorhaz was killed from within while underground... with the barbarian still in its stomach...

Yes.

You could rule that the thing starts taking acid damage too if an attack hits hard enough 

No multi attack though... So that helps? 

my party are currently fighting one wish me luck!

Lel my dm just gave me one of these as a pet 

This things insane even with a lv 15+ 6 man group

It's cool that frost giants tame these things considering that they are way stronger than any individual frost giant.

Ok so just curious, say a party is fighting this thing and in the party is a Druid who is at a lvl high enough to wild shape into large creatures(yes Druid has circle of the moon). Let’s say by chance said Druid ends up on the tail end of an ambush and gets swallowed. What happens to the Remorhaz if the Druid Wild Shapes into a large animal while swallowed? The creatures stat block says medium or smaller can be swallowed so would the Druid be spewed up because it’s to large? What would happen?

YES IT WOULD

We're a level 4 party and had to face this. Let's just say. Ouch

Killed my druid

whats the plural. rhemorazes, rhemorazs, rhemorazi? this is my most important question.

These guys can be rough, especially in melee. Range, range, range is all I can say!

Maybe remorhaz is the plural and remorhazi is singular like Djinn and Djinni

3d6 every hit

I'm about to paint a pair of these tonight on YouTube live. I have the Nolzur's Marvelous Miniatures adult remorhaz and the Gale Force 9 D&D collector's series one that is so much bigger. 

I just assembled the Gale Force 9 tonight, magnetized it to its gargantuan base and magnetized its head to its neck.

I run Eberron 3.5, so I was looking at its entry in the monster manual that puts it at CR7 back then; however it didn't cause the fire damage when you struck it with melee weapons only if you struck it with an unarmed attack or with natural weapons. However, however it still dealt the fire damage to you when it, hits you.

It also had 7d10+35 (average 73hp) instead of 5e's 17D12+84 (avg 195hp) for hit dice.

If you advanced its hit dice, it maxed out at 21d10+105 (315 maxed).