In this work, an inverse forced convection-radiation boundary design problem for a two-dimensional channel with a forward and a backward-facing step is solved. In design problems, it is desired to provide uniform temperature and heat flux distribution over a design surface by obtaining the unknown temperature distribution over a heater surface. The flow is laminar, and the high-temperature gas is treated as an absorbing, emitting and isotropic scattering medium. The channel walls are considered to be diffuse-gray absorbers and emitters. The radiative transfer equation is solved by the discrete ordinates method. The conjugate gradient method which is based on an optimization technique is applied to solve the inverse problem. By the applied numerical method, the desired heat flux distribution over the design surface is very well reconstructed. Also, the effects of step inclination angle, scattering albedo and optical thickness on the solution of the present inverse problem are explored. It is found that by increasing in step inclination angle, scattering albedo and decreasing in the optical thickness, the total heat flux over the heater surface increases.