In this study, the combustion model of KIVA-3V code has been modified for considering the coupling of flow field computations with a detailed chemical kinetic scheme which involves the multi-reactions equations and the CHEMKIN software has been coupled for solving the chemical reactions and calculation of heat release from fuel in each control volume considered in flow field. The HCCI engine conditions have been used and the effect of parameters such as the swirl at the start of compression stroke, initial temperature, initial pressure, engine speed, compression ratio and equivalence ratio on the combustion characteristics of a n-heptane fuelled HCCI engine have been studied. The chemical kinetic scheme used here, consists of 52 elementary reactions and 29 species which involves the reactions required for calculation of NOX also. The results show good agreement with available experimental data.