Landfill gas (LFG) is a waste-product resource composed primarily of methane and carbon dioxide that can be collected and used to produce power either by extracting the methane or using the landfill gas directly in an internal combustion engine in what amounts to a net-negative greenhouse gas emission process. The carbon dioxide component of LFG dilutes the fuel and absorbs some of the heat of combustion, but also suppresses knock. A model is developed, using KIVA-4 code, to simulate engine performance at various operating conditions and evaluate the benefits of methane purification and direct use of LFG as a fuel.It was found that landfill gas used directly at higher compression ratios than can be used for pure methane fuel produces higher fuel efficiency than can be achieved using pure methane. Furthermore the study has shown that the fuel efficiency of the engine can be achieved within 5% of that of pure methane fuel for landfill gas, diluted up to 1:1 with carbon dioxide by simply adjusting the spark timing of the engine. A model for combustion duration is also developed which can estimate the combustion duration in an internal-combustion engine for varying fuel compositions.