A gas-heated steam reformer (GHR) converting natural gas to synthesis gas of a composition typical for methanol or Fischer-Tropsch synthesis was modeled at steady state. The model is 2D comprising a fixed-bed reactor model and an annulus model, where the latter represents an annular space on the shell side of the GHR. The heterogeneous reactor model is presented here, with particular emphasis on the mass and heat transfer limitations between the gas and the pellets. Several correlations for the mass and heat transfer coefficients were evaluated, and it was found that the interphase transport limitations were important for evaluation of local conditions and need to be taken into account when studying reaction rates and interpreting these to obtain kinetic rate data. The interphase transport limitations had, however, negligible effect on the overall reactor simulation results. It was also found that the point in the reactor where the shift reaction changes direction is moved significantly when interphase transport limitations were incorporated.