The evaluation of about 25 years of Doppler data has shown an anomalous constant deceleration of the deep space probes Pioneer 10 and 11. This observation became known as the Pioneer anomaly (PA) and has been confirmed independently by several groups. Many disturbing effects that could cause a constant deceleration of the craft have been excluded as possible source of the PA. However, a potential asymmetric heat dissipation of the spacecraft surface leading to a resulting acceleration still remains to be analysed in detail. We developed a method to calculate this force with very high precision by means of finite element (FE) modelling and ray tracing algorithms. The elaborated method is divided into two separate parts. The first part consists of the modelling of the spacecraft geometry in FE and the generation of a steady state temperature surface map of the craft. In the second part, this thermal map is used to compute the force with a ray-tracing algorithm, which gives the total momentum generated by the radiation emitted from the spacecraft surface. The modelling steps and the force computation are presented for a simplified geometry of the Pioneer 10/11 spacecraft including radioisotope thermoelectric generators (RTG), equipment/experiment section and the high gain antenna. Analysis results how that the magnitude of the forces to be expected are non-negligible with respect to the PA and that more detailed investigations are necessary. The method worked out here for the first time is not restricted to the modelling of the Pioneer spacecraft but can be used for many future fundamental physics (in particular gravitational physics) and geodesy missions like LISA, LISA Pathfinder or MICROSCOPE for which an exact disturbance modelling is crucial.