Komodo dragons have a specialized gum tissue structure that breeds over a dozen identified varieties of bacteria... a bite from a Komodo is certain death by infection, and how they kill prey that is too big for them to fight... it wanders off, dies, and then they go looking for it.
See, THIS is Nature's way of problem solving! Keep it simple stupid!
The evolution of flame is so outrageously improbable for the exact reasons oneironaut stated.
It would be more likely (still not likely... but MORE likely, lol) that they could FART fire, lol... at least methane (flammable hydrocarbon, exists as a gas at body temperature) is already a byproduct of digestion... hahahahaha I can just imagine St. George battling his big ol' dragon, and the thing turns around and RIPS one at him....