Natural gas is composed primarily of methane, so the major elements are carbon and hydrogen. Almost all other compounds are removed (and used for something else) from raw natural gas before it is burned as a fuel, except they deliberately add traces of mercaptan, which contains sulfur, to give the gas its characteristic smell; methane by itself is odorless and in case of a leak could build up to flammable or explosive levels.
Methane is highly flammable (CH4 + [2]O2 = CO2 + [2]H2O). If it leaks out into the air it will generally dissipate and decompose, however if it is confined (as in a house) and reaches levels of about 5 to 15% of the air, it will become explosive; a spark in that house may level it..