A model for a methanol reformer for automotive applications is presented. The steam reforming reactor is coupled with a burner that supplies heat. In the burner, methanol combustion takes place both homogeneously (in the gas phase) and heterogeneously (within a platinum catalyst layer deposited on the heat-exchange surface). The steam reformer is modeled as a pseudo homogeneous reactor. The reforming kinetics is validated by comparing theoretical results with experimental data found in the literature. The pressure inside both reactors is assumed to be constant,, and the gas is in plug flow. The model consists of 19 partial differential equations and is solved numerically. An optimal reactor design is proposed to achieve maximum reactor compactness and methanol conversion both in the reformer and in the burner, as well as a temperature in the reformer always lower than 573K to avoid catalyst sintering. Furthermore, the reactor dynamics is analyzed for the sake of defining operability maps in terms of gas velocities of the feed streams, which are expected to support the choice of an optimal reactor control strategy to avoid methanol leakage and thermal "runaways".