An apparatus, system, and method are disclosed for determining a catalyst bed temperature. The method may include receiving a current fluid mass flow within an exhaust pipe and a total amount of unburned fuel available for catalytic combustion. The method may further include determining a plurality of fluid enthalpy values corresponding to various locations within the exhaust pipe. The method may continue with calculating an amount of fuel burned on an upstream catalyst based on the plurality of fluid enthalpy values, and determining an amount of fuel remaining that is available for combustion on a downstream catalyst. The method may determine a combustion efficiency of the downstream catalyst based on the temperature rise across the downstream catalyst, and an amount of heat generated within the downstream catalyst based on the combustion efficiency and the amount of fuel remaining that is available for combustion on the downstream catalyst. The method may conclude with determining a downstream catalyst bed temperature based on the net heat generation, the fluid mass flow through the downstream catalyst, and the thermal response of the downstream catalyst.