Multidimensional computations were made of combustion of natural gas engine via the KIVA-II code to evaluate the combustion and emission characteristics. In the combustion submodel, a two-step kinetic reaction mechanism is employed to account for oxidation of methane. The first of the two global rate equations controls the disappearance of methane, and the second, the oxidation of carbon monoxide. Four types of combustion chamber and a two-spark-plug geometry are considered to achieve quick flame propagation of the lean air-methane mixture. The effect of spark plug locations on the combustion processes is discussed. The calculated results show that the more effective burning process with lower NOx emission could be achieved by proper design of the geometry of the piston bowl and the arrangement of the spark plug by matching of the flame front development with the in-cylinder gas flow. The computations indicate further that adoption of the two-spark-plug geometry could shorten the combustion duration by 53% for an extra lean air/methane ratio of 25.0.