Ok this is about a conversation I had with ihavenocluewhattodo a while back (one of the VERY few intelligent conversations we've had)

He knows more about dinosaurs and such than me (I FUCKING HATE IT!!!)

Sorry for the outburst it's just annoying to me not knowing more than him.

Anyway to the theory

Ok so they would have two stomach like organs one for food and one for methane. And the food one would be below the methane one so that they didn't have to worry about the methane being contaminated.

Before the fire would come up the esophagus (tube to stomach for all of you that don't know medical shit) it would have to be made in the first place. Well this part was a idea that I came up with. The dragon would create pressure and it would combust causing the fire and the pressure behind it would cause it to move up the esophagus. 

Ok so now you're asking Hmm wouldn't that burn them? Well again medical knowledge. There would be a mucus (snot for all you idiots) layer covering the tender flesh along the esophagus walls.

So long story short it's possible and dragons could've been real.