Dual-fuel combustion provides a relatively easy and inexpensive alternative to conventional diesel engine combustion by drastically reducing fuel consumption with comparable performance characteristics. Accurate simulation of the dual-fuel combustion requires utilization of a detailed chemistry combined with a flow simulation code. In the present study, the combustion process within the diesel and diesel/gas dual-fuel engine is investigated by use of a coupled 3D-CFD/chemical kinetics framework. In this study, methane and n-heptane are used as representatives of the natural gas and diesel fuels. The multi-dimensional KIVA-3V code, with modified combustion and heat transfer models, incorporates a chemical kinetics mechanism for n-heptane and methane oxidation chemistry. The source terms in energy and species conservation equations due to chemical reactions are calculated by integrating the CHEMKIN chemistry solver into the KIVA code. The model is applied to simulation of a medium duty dual-fuel converted diesel engine. A chemical kinetics mechanism which consists of 42 species and 57 reactions is used for prediction of n-heptane oxidation chemistry. Simulation of dual-fuel combustion is performed using the same mechanism with addition of a series of major methane oxidation pathways. The results show that Zheng and Yao's n-heptane mechanism which had been previously validated in their work, can model the diesel and dual-fuel combustion, where fuel-rich zones are present. The predictive model of this study is validated using available published experimental data. Results show that pressure and ignition delay predictions are in good agreement with experiments. Based on constant total mixture input energy in dual-fuel combustion, increasing pilot fuel amount leads to shorter ignition delay and peak pressure increment. It is found that concentrations of NOx and CO emissions tend to increase at higher pilot fuel injection quantities.