This study mainly focused on the combustion mechanisms of natural gas (of major constituent were methane, ethane etc) in an Internal Combustion Engine (ICE). Both detailed & reduced mechanisms were implemented in CHEMKIN 4.1.1 for 0-D closed internal combustion (IC) engine module. In present study, two detailed (GRIMECH 3.0 & UBC MECH) and two reduced (One step & four step) kinetic mechanisms were tested on three engine geometries. The combustion due to these mechanisms was characterized by plotting various parametric profiles like temperature, pressure and major species. Three mechanisms ((4-step, GRIMECH3.0, & UBC MECH) successfully showed combustion while one step global mechanism was unable to predict any combustion reaction. Detailed mechanisms predicted the maximum pressure (atm) of 70, 40 and 30 for each of design I, II & III respectively. The maximum chamber temperature with these mechanisms was predicted above 2000 K. Four step reduced mechanism predicted maximum cylinder pressure of 90 atm for design II & III and 300 atm for design I. Maximum cylinder temperature was of 7000 K for Design I and 5000 K for each of design II & III respectively. GRIMECH successfully predicted the formation of pollutants like CO & NOx while 4-step mechanism only predicted CO and unable to predict NOx formation. Although, there were some discrepancies among the simulation results for each of the engine design but on the whole it can be concluded that GRIMECH3.0 mechanism was favorable & successfully predicted the pollutants formation during combustion in the IC engine chamber.