Density of Smaug
If Smaug can sink in molten gold -- which has a density of 17,310 kg/m^3 -- how on Middle Earth can he fly?
Solid steel can float in mercury -- which is rather less dense than gold at 13,534 kg/m^3.
Smaug should have floated on that pool of gold like a happy, overheated, scaly duck.
Badass Buttocks of Badassitude
Bombur can work bellows with greater than several horsepower output using nothing more than his gluteus maximus to propel himself upward.
Turn the lights off on your way out
Before the dwarves were forcibly evicted, the furnaces were operating. We know this because the gold in the crucible was not chunks, but instead a solid mass from being previously melted.
It's possible the dwarves had coal or coke for fuel, but more likely that they had managed to harness natural gas. As certain natural and man-made disasters have shown us, underground sources of gas can burn almost indefinitely -- possibly even the 171 years that Smaug held Erebor.
I'm going to ignore the obvious problem of ventilation: the dwarves had build an entire civilization under a mountain, and they all had to breathe.