By means of steam reforming, natural gas is converted to carbon dioxide and hydrogen. The reactions take place in reactor tubes which are covered with catalyst at the inside, where the reactive mixture flows. At the outside they are heated by combustion of natural gas with air. In this paper the conversion process is modeled for a single reactor tube by both chemical reaction models and heat transfer models. The model yields data of temperature, heat transfer and concentrations of hydrogen, carbon monoxide and natural gas along a reactor tube. Simulated temperatures have been validated with measured data from a prototype reformer. Next, the model was used to evaluate the performance of the reformer for six design modifications. It was found that the most promising improvements are created by increasing the air fraction in the burner and the thickness of the insulation shield.