[thackney]

CO2 is twice as heavy as air

But methane is not. It is lighter than air and an accumulation of it in a cloud would be far above the surface.

[Fred Hayek]

Methane MW (molecular weight) 16
Air avg MW 29
Carbon Dioxide MW 44
Under standard conditions (1 atm, ~20 Celsius), ideal gas rules are good approximation so 1 mole volume (22.3 liters) weighs the molecular weight in grams.