The use of Compressed Natural Gas (CNG) has demonstrated the potential to decrease Particulate Matter (PM) and nitrogen oxide (NOx) emissions simultaneously when used in a dual-fuel application with diesel fuel functioning as the ignition source. However, some authors do find that NOx emissions can increase. One postulation is that the conflicting results in the literature may be due to the difference in composition of natural gas around the world. Therefore, in order to investigate if CNG composition influences combustion performance and emissions, four unique mixtures of CNG were tested (i.e., 87% to 96% methane) while minimizing the combined difference of the density, heating value, and constant pressure specific heat of each mixture. This was accomplished at moderate energy substitution ratios (up to 40%) in a single cylinder engine operating at various loads. Previous analysis of these results did not reveal noticeable macroscopic trends with respect to the relative amounts of methane, ethane, propane, and isobutane. Thus, a statistical analysis using 2-way and 1-way Analysis of Variance along with Pearson correlations was performed to determine if dependencies exist between the results and the composition of CNG. It was found that the loading and substitution rate dominated the results with a relatively small influence noted from the amount of methane on the hydrocarbon emissions. As a result, the CNG composition normalization procedure employed may provide a simple methodology of ensuring consistent performance and emission results from different CNG compositions when used in a dual-fuel manner with diesel.