So, basically Fischer Troph process, just with an insanely poor feedstock gas. You can make urea from air and water, but you really have to remember air is 78% nitrogen, and after oxygen the rest of the gas concentrations is so low it is not a worry. Now, if you are using methane and such, you really need a large source that is constant, large volume and will be reliable in supply, like a gas field. A garbage dump is really going to be low volume, irrespective of size, and using CO2 from a fossil fuel burning power plant is a short step to insanity, those pesky laws of thermodynamics, and no catalyst is going to magically make the separation of CO2 back into C and O2 work without putting in more energy that what the reaction released in the first place.
Law of entropy, it always gets worse, and you never can break even.