Fired Heaters are easily the most energy-extensive units in process industry. Often times, the operating cost of Fired Heaters can amount to almost 20-50% of a total plant’s operating cost. Monitoring the efficiency of these heaters and continuous improvement in design is critical. Significant advancements have been made in simulating the temperature distribution inside a Fired-Heater over the past decades. This paper focuses on simulating the Temperature distribution inside the Radiant Section of a Fired-Heater using the Long-Furnace model; a more accurate extension of the Lobo-Evans method widely accepted in industry. A novel mathematical model is developed and applied to a Fired-Heater in the Syrian Refinery. The resulting MatLab simulation shows a one-direction temperature distribution along the radiant chamber. This is the first academic approach of its kind. The model is computationally less extensive than a CFD approach. This model can provide future Fired-Heater designers a preliminary basis for comparison without having to go through extensive CFD calculations.