Thermal radiation is known to play a greater role in opposed-flow flames in the absence of gravity, where the lack of buoyancy-induced natural convection allows for very low velocity flows. Understanding the role of radiation in microgravity flame spread is important for spacecraft fire safety but performing experimental work in microgravity is difficult and costly. While radiation primarily acts as a heat loss mechanism, radiation feedback from the gas to the solid surface may pre-heat the fuel and promote flame spread. A numerical flame spread model is employed in this work to simulate flames in microgravity and normal gravity with a comprehensive radiation model with parameterized flow velocity, oxygen level, and fuel geometry. The radiation model is capable of including or neglecting each aspect of radiation heat transfer: gas losses, surface losses, and gas-to-surface feedback. By comparing the results of the radiation models in flames spreading over the entire range of opposed-flow velocities from quenching to blow-off, the effects of each mode of radiation can be isolated and quantified over the radiative, thermal, and kinetic regimes. It is found that radiation is most significant in the radiative regime, with the five models converging as the opposed-flow velocity reaches the kinetic regime. Gas radiation is the primary radiation mechanism for describing the gaseous flame and its importance increases at elevated oxygen levels. Surface radiation is important for describing the state of the solid fuel, including vaporization temperature, but its importance diminishes as burnout reduces the length of the radiating solid fuel. Radiation feedback never exceeds 10% of the heat transfer and provides limited pre-heating but is critical in describing radiative regime flames and continues to minorly enhance flame strength in the thermal and kinetic regimes. Downward spreading flames under normal gravity are found to behave similar to kinetic regime flames in microgravity where radiation has become less impactful. Wider fuels are found to significantly enhance radiation feedback, which increases microgravity spread rates and agrees with limited experimental results. Fuel thickness is found to be inversely proportional to spread rate, though disagreement exists between the experimental and computational radiation intensity profiles.