In light of the energy transformation process initiated by the Paris Agreement in 2015 and the formulation of the goals in the European Green Deal of 2019, the utilization of nuclear energy becomes a strategy to achieve the target of reduction of greenhouse gases (mainly CO2). According to the Strategy for Responsible Development, Poland plans to introduce nuclear power into the energy system sector and build research and commercial nuclear reactors of various types. The presented work was realized as part of the scientific national GOSPOSTRATEG-HTR project, which was established to develop the novel pre-concept of a 40MWth research nuclear High Temperature Gas-cooled Reactor (HTGR). The extensive calculation campaign was carried out using Ansys Fluent (Computational Fluid Dynamics) and MELCOR (integrated, engineering-level system) simulation software, combining the best features of both representative approaches. The studies covered the steady operation of the HTGR core on the 400th day of fuel campaign. The power distribution came from outsourced neutronic analysis (conducted within the same project). The core consists of prismatic graphite fuel blocks in which the cooling (helium) and fuel (UO2 with graphite) channels were located. CFD computations were performed for one twelfth of the reactor core layer (height of one fuel block), while for MELCOR the primary and secondary sides were modelled. Comparison of the results concentrated on maximum temperature values in axial and radial directions, average temperature values in cooling channels, bypass, fuel, and graphite. The most important added values of the presented research are ready-to-use tools and methodologies for the analysis of HTGR, knowledge of limitations of CFD and MELCOR approaches, their use in a complementary manner, a deep understanding of the thermohydraulic processes occurring in HTGR and experience in such analysis.