The current trend of downsizing used in gasoline engines, while reducing fuel consumption and CO2 emissions, imposes severe thermal loads inside the combustion chamber. These critical thermodynamic conditions lead to the possible auto-ignition (AI) of fresh gases hot-spots around Top-Dead-Center (TDC). At this very moment where the surface to volume ratio is high, wall heat transfer influences the temperature field inside the combustion chamber. The use of a realistic wall temperature distribution becomes important in the case of a downsized engine where fresh gases hot spots found near high temperature walls can initiate auto-ignition. This paper presents a comprehensive numerical methodology for an accurately prediction of thermodynamic conditions inside the combustion chamber based on Conjugate Heat Transfer (CHT). The fundamental bricks for the simulation are well described: combustion modeling, CHT methodology and modeling strategy, i.e the choice of the computational domain with its appropriate boundary conditions. Conjugate Heat Transfer is solved by means of a coupled simulation between the fluid involved in the combustion and the solid engine-head and valves. Heat transfer through the fluid/solid interfaces are well captured and used to solve for the solid. Then, the resulting surface temperature distribution is used as boundary condition to solve for the fluid. The CHT is succesfully applied to the Renault H5Ft downsized direct injection engine operating at 2500 rpm. On the fluid side, the combustion simulation is validated in comparison with the experimental mean in-cylinder pressure curve. On the solid side, wall temperature values measured with thermocouples are used to assess the accurate prediction of the temperature distribution along the gas-exposed surfaces.