Hmm.. I'm not sure I belive it's "adding carbon..at a slower rate" but I'll have to think a little more deeply about the ecocycle and see if it adds up. This is sort of like "environmental accounting" which is a new method of applying double-ledger bookkeeping to chemicals used in manufacturing processes. If the books don't balance, if the quantity of chemicals-in doesn't exactly equal chemicals in the final products, you're leaking pollution into the environment. It's a brilliant idea.
MMD, I didn't know that city gas was methane, I thought it was LP gas. See, there's already a methane infrastructure in place!