For a more extensive investigation of polymers, a better understanding of its gasification process is extremely essential. Especially compared with the extensive studies in the gas phase, the solid phase received rather little attention. Thus, the purpose of this paper is in its comparison of several modeling approaches for thermal degradation and systematic demonstration of the effects of basic assumptions on the model results, while taking account of in-depth radiation. For this object, this study has examined in more detail the degradation for a horizontally positioned polymer which is exposed to external radiation. After a preliminary study, three different solid degradation models were chosen from the literature, and their corresponding mass and energy conservation equations and boundary conditions were assembled. The in-depth non-gray as well as gray radiation was taken into account by solving relevant radiative transfer equation. In order to concentrate on the fundamental mechanisms of polymer degradation, gas phase reactions and subsequent heat feedback from the gas were neglected. This corresponds to a nitrogen environment. Various other parameters such as the polymer refractive index, polymer absorption coefficient, convective heat loss, solid fuel thickness and external radiative heat flux were changed to discuss their effects on the quantitative as well as qualitative change in the mass loss rate. While the effect of the solid fuel thickness on the mass loss rate was negligible for a thick sample, each parameter among the polymer refractive index, polymer absorption coefficient, convective heat loss, and external radiative heat flux incurred a non-negligible change in the results. Furthermore, the convection term in the energy equation, which is usually neglected in the in-depth pyrolysis by many other works, was shown to account for 29% decrease in the mass loss rate. Finally, depending on the solid degradation model used, the addition of gray radiation to the energy equation was shown to augment or diminish the mass loss rate. It was further enhanced by taking account of the non-gray radiation. This results from the fact that the inner solid is more quickly heated due to the far-reaching effects of radiation from the surface.