The thermal behaviour of spacecraft especially of manned ones is governed by various thermophysical effects. Heat transfer by radiation, conduction and convection influence the overall behaviour of the spacecraft. Energy production, distribution and consumption have also strong impacts on the temperature field prevailing in the spaceplane. Moreover, chemical reactions, metabolic rates of the crew and external heat loads determine the thermal behaviour of the spacecraft. For design and optimisation of the system and its subsystems a mathematical model is required which describes these effects.There are many tools available with which parts of the above mentioned aspects can be covered. However, a comprehensive model of a spacecraft reflecting all aspects can only be established with a combination of different simulation tools.In the present paper methods are described, how different simulation tools can be used to set up such a model. Some of them are used during the definition and set up of the models. Others are required during the execution of the calculations. As an example, the procedure to establish the Thermal Mathematical Models for the Pressurized Volumes of HERMES is described.During setup and establishment of the ESATAN model the radiative heat transfer between the inner surfaces was studied. At the same time a CFD study of the flow field in the volumes was carried out. The resulting node model is then connected with a modular system simulation of the ECLSS to get a higher level model in which both, the lumped parameter modelisation of the pressurized volumes as well as the component based model are combined and iteratively processed.Steady state and transient loadcases have been calculated and gave excellent results.