ESA has commissioned the Crystal Growth Laboratory in the framework of the Materials Science Laboratory (MSL) User Support Program to develop the MSL furnace inserts and sample-cartridge assemblies Thermal Modeling Tool (TMT). The TMT assists the definition, preparation and analysis of the experiments onboard the International Space Station (ISS). The tool is based on the CrysVUn software, which was especially designed for global simulation of heat and mass transport processes during crystal growth and alloy solidification in high temperature furnaces with complex (axi-symmetric) geometries. The main features of CrysVUn are briefly presented in this paper. The preliminary thermal model of the Low Gradient Furnace is illustrated and the model optimization strategy using genetic algorithms is briefly explained. Initial correlation results show that good agreement between simulated and measured axial temperature distributions is obtained. Direct modeling of the radiative heat transfer through the heater Multi-Layer Insulation (MLI) using view factors appears to be necessary for an accurate prediction of the resulting heater power. An example for the application of the TMT is presented.