We present results from radiation-hydrodynamics calculations which show the central role resonant self-absorption plays in reducing radiative energy loss rates in high-gain ICF target chamber plasmas. Calculations were performed using a non-LTE radiative transfer model which we have recently coupled to our target chamber radiation-hydrodynamics code. The lower radiation fluxes escaping the plasma, which occur due to the self-absorption of line radiation in their optically thick cores, lead to significantly lower temperature increases at the surface of the target chamber first wall. The calculations were performed for the SIRIUS-P laser-driven direct-drive ICF power reactor. In this conceptual design study, high-gain targets release approximately 400 MJ of energy in the center of a gas-filled target chamber. The target debris ions and x-rays are stopped in the gas, and the energy is reradiated to the chamber wall over a much longer time scale. Because the time scales are comparable to the time it takes to thermally conduct energy away from the first surface, the thermal stresses and erosion rates for the first wall are greatly reduced.