<p>Accurate modeling of facility effects in ground testing of electric propulsion (EP) requires reliable descriptions of gas-surface interactions in vacuum chambers. However, the gas-surface accommodation coefficients that govern energy and momentum exchange during wall reflection are poorly characterized under actual EP testing conditions. This study proposes an experimental-numerical approach to infer these coefficients by combining rarefied-gas flow measurements with direct simulation Monte Carlo (DSMC) simulations. In this approach, a Patterson-type rarefied-gas probe is used to measure the angular distribution of gas flux reflected from a target wall in a vacuum chamber. DSMC simulations then replicate the experimental environment and determine the best-fit tangential-momentum and normal-energy accommodation coefficients by sweeping these coefficients and matching the measured flux patterns. The approach quantitatively reproduces the observed reflection patterns and provides a practical means to estimate gas-surface interaction parameters directly in test facilities. As a demonstration, the method was applied to xenon reflected from a buff-polished aluminum surface under EP-relevant vacuum conditions. High-accommodation cases best matched the measured angular flux patterns within the tested parameter range. These results demonstrated a new practical framework for estimating gas-surface parameters under controlled vacuum-chamber conditions, providing a foundation for future predictive chamber modeling.</p>