A method of optimizing steam methane reforming in a multiple cell, down-fired furnace. The method includes the steps of introducing hydrocarbons and steam to a plurality of catalyst filled process tubes, combusting gases through a plurality of center cell down-fired burners at a burner discharge velocity of V m/s, and a mass flow rate of 1 kg/s, combusting gases through outer cell down-fired burners at a burner discharge velocity of 1.5V-2V m/s and a mass flow rate of 0.5-0.65 kg/s, and finally discharging combustion products from the outer cell down-fired burners and the center cell down-fired burners at a substantially uniform momentum.