A numerical model is presented in this work to compute the inter-dependent fields of flow, temperature and the concentrations of multiple gases in a single tubular solid oxide fuel cell (SOFC) system. It was supposed that the fuel gas supplied to the fuel cell is from a pre-reformer and thus contains hydrogen and proportions of carbon monoxide, carbon dioxide, steam, and methane. The model takes mixture gas properties of the fuel and oxidant as functions of the numerically obtained local temperature, pressure and species concentrations, which are inter-dependent and intimately related to the electrochemical reaction in the SOFC. In the iterative computation steps, local electrochemical parameters were simultaneously calculated based on the local parameters of pressure, temperature, and concentration of the species available at each step. Upon the convergence of the computation, both local details and the overall performance of the fuel cell could be obtained. The numerical results obtained are helpful for better understanding of the operation of SOFCs.