Abstract A 3-dimensional non-isothermal simulator comprising a high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) and a methanol steam-reforming cell (MSR-C) was developed in Fluent (Ansys™). The simulator takes into account most of the significant physical processes, including the electrochemical reactions and carbon monoxide poisoning effect on the electro-catalytic activity of the FC; it also considers the methanol steam reforming (MSR), water gas shift (WGS) and methanol decomposition (MD) reactions in the MSR-C. The developed model for the integrated MSR-C/HT-PEMFC unit was simulated between 443 K and 473 K and validated with experimental results reported in the literature, showing always a very good agreement. The thermal sustainability of the MSR-C/HT-PEMFC unit was assessed, and the role of the thermal insulation and air intake (cathode) stoichiometry in the thermal equilibrium of the device were analysed. A novel integrated MSR-C/HT-PEM stack with ten cells was proposed and simulated, showing a performance above the reported in the literature for similar devices. The results indicated that the proposed stack operates at currents between 4.5 A (0.1 A cm −2 ) and 54 A (1.2 A cm −2 ) without any external heat source. To minimize the degradation of the components the stack should adapt the operating temperature to the current density.