A larger use of natural gas in internal combustion engines will reduce the dependence upon petroleum imports. However, large variations in gas composition as a result of the gas origin and/or gas treatments applied can affect engine performance and emissions. This study used a three-dimensional computational fluid dynamics internal combustion engine model (validated with experimental data) to investigate the effect of adding heavier hydrocarbons (ethane and propane) to the gas mix. The simulation investigated the effects of spark timing, intake pressure, equivalence ratio, and engine speed when the engine operated with six different natural gas blends (methane number from 50 to 100 and Wobbe index from 48 to 58) that had a nonlinear relationship between the methane number and the Wobbe index. The results showed that a lower methane number and a higher Wobbe index increased and advanced in-cylinder pressure, advanced the combustion phasing, decreased the ignition lag, and shortened the combustion duration, regardless of the operating condition as a result of the higher laminar flame speed and energy density. In addition, it increased nitrogen oxides emissions but lowered unburned hydrocarbon emissions. A higher sensitivity to gas composition was found when the methane number was lower than 70. Moreover, the results suggest that gas composition will not affect lean-burn operation if the methane number is above 70. This suggests that the engine control system of a diesel engine converted to lean-burn natural gas spark-ignition operation will adjust to changes in natural gas composition as long as the methane number of the gas is at least 70.