Reformer gas (syngas) addition to main fuel is a practical solution for combustion timing control in HCCI engines. This study emphasizes the understanding of various effects of reformer gas (RG) addition, with composition of 75%vol H-2 and 25%vol CO, in HCCI combustion by developing an artificial inert species method and using a detailed chemical kinetics multi-zone model. Three fuels (iso-octane, n-heptane, and natural gas) with different autoignition characteristics were used in this study. The developed multi-zone model was validated for mentioned fuels at various percentages of RG using six experimental cases of a single-cylinder CFR engine. The results showed that increasing reformer gas fraction in the fuel mixture advanced methane fuel combustion timing, retarded the combustion of n-heptane and had insignificant effect on iso-octane combustion. Thermal effect of RG in all fuels resulted in earlier start of combustion (SOC) because of the mixture specific heat ratio enhancement. The SOC for methane and iso-octane fuels was advanced by RG addition due to the chemical effect of RG. However, the chemical effect of RG for methane fuel is more significant. By adding 30% RG to methane fuel, H-2 advanced the SOC by -5.6 degrees CA and CO retarded it by 1.4 degrees CA due to chemical effect and also the interaction effect of CO and H-2 advanced the SOC by -1.8 degrees CA. For iso-octane fuel, adding 30% RG, both CO and H-2 advanced the SOC by 0.4 CA due to their chemical effect. On the other hand, in n-heptane fuel, H-2 is almost responsible for all of the chemical effects of RG for retardation of SOC.