Maybe natural gas combustion doesn't produce enough carbon dioxide? Just a guess; I'm not a chemist.

Okay, it looks like NG is mostly methane, which is CH4. Propane is C3H8. Combusting those yields:

CH4 + (2)O2 -> CO2 + (2)H2O
C3H8 + (5)O2 -> (3)CO2 + (4)H2O

It says on the MM website that butane can also be used, so:

(2)C4H10 + (13)O2 -> (8)CO2 + (10)H2O

Now that I've written all that out, I still don't know what it means. Maybe NG just burns too readily or something.