A 1-D stationary reactor model describes the catalytic conversion of a gaseous hydrocarbon fuel with air and steam to synthesis gas by autothermal reforming and catalytic partial oxidation. The model defines two subsequent sections in the reactor: an upstream oxidation section and a downstream reforming section. In the oxidation section, all of the oxygen is converted, with partial conversion of the fuel. An empirical fuel utilization ratio is used to quantify which part of the fuel is converted in the oxidation section as a function of the relative flows of air and steam. In the oxidation section, the gas temperature rapidly increases toward the top temperature at the intersection with the reforming section. In this section the temperature decreases while the fuel is further converted with water and CO2 as oxidant. For methane as fuel, simulation results are compared with experiments. For multicomponent fuels such as natural gas and naphtha, application of the two-section model is described.