The heat transfer will be by:radiation and conduction.If temp is low and does not permit radiation it will be purely conduction

You can use S2S model in fluent

your case study depends on the boundary conditions of the top and bottom surfaces  and the limitation of  heating and cooling  and the type of gas in order to determine the expected heat transfer types conduction , convection or radiation and which model to be used.

also your case study unclear maybe due to high flux lead to high pressure.