High-pressure direct-injection (HPDI) natural gas engines continue to draw much attention for the highly efficient and clean combustion process compared to traditional diesel engines. However, the real ignition processes of HPDI natural gas engines are very different from the ignition processes of n-heptane/methane/air mixtures. In this work, the ignition of homogeneous n-heptane/methane mixtures in a constant-volume chamber under HPDI natural gas enginelike conditions is numerically studied. Results show that the temperature and equivalence ratio have a significant effect on ignition delays over a wide range of conditions, whereas the effects of the pressure and the methane mass fraction are only profound at low temperatures. A small number of intermediate species almost does not influence the effects of various factors (temperature, equivalence ratio, pressure, and methane mass fraction) on the ignition of the fuel blends. With more intermediate species, the effects of equivalence ratio on the ignition of the fuel blends are further mitigated. Based on the calculated results, ignition delay regions were categorized into four major regions by the equivalence ratio and methane mass fraction in fuel/air mixtures. A small number of intermediate species almost does not affect the four regions. However, with more intermediate species, the four regions are very different from those of fuel/air mixtures. The sensitivity analysis indicates that intermediate species can significantly reduce the effects of n-heptane-related reactions. Rates of production (ROP) analyses show that a small number of intermediate species almost has no effect on ROPs, whereas more intermediate species mainly affect not only the ROPs but also the temperature. Reaction path analysis shows that the intermediate species have a significant influence on the reaction paths of both n-heptane and methane. This work can provide a theoretical basis for further investigation of the ignition control of HPDI dual-fuel engines.