This paper describes a model for the simulation of the joint operation of internal combustion engine (ICE) with methanol reformer when the ICE is fed by the methanol steam reforming (SRM) products and the energy of the exhaust gases is utilized to sustain endothermic SRM reactions. This approach enables ICE feeding by a gaseous fuel with very favorable properties, thus leading to increase in the overall energy efficiency of the vehicle and emissions reduction. Previous modeling attempts were focused either on the performance of ICE fueled with SRM products or on the reforming process simulation and reactor design. It is clear that the engine performance is affected by the composition of the reforming products and the reforming products are affected by the exhaust gas temperature, composition and flow rate. Due to the tight interrelations between the two main parts of the considered ICE-reformer system, it is desirable to create a single model that simulates joint operation of the ICE and the SRM reactor. Such a model is built with the GT-Power software. It employs published catalytic reactions kinetics. The reformer model is validated using experimental results from a small scale model reactor. The developed model can be used for the performance optimization of the whole ICE - reformer system including design of the reactor. Simulations with a reformer bed geometry of counter current, multiple tubes performed using the developed model show that heat-transfer is limiting the reformer size, requiring 1.5 m2 for complete methanol conversion in lean operation of a 75 kW ICE. Moreover the model can be applied to predict a transient behavior of the system owing to the time dependent approach implemented in the reformer and engine calculations.