In this study, we developed a modular and robust platform for the late-stage alkylation of heteroarenes using readily available gaseous alkanes as C1-C4 radical donors. By employing hydrogen atom transfer (HAT) photocatalysis under continuous-flow conditions, we demonstrated efficient alkylation of pharmaceutically relevant heterocycles, including quinolines and marketed drugs. The method overcomes traditional challenges associated with gaseous alkanes, such as their high bond dissociation energy and poor solubility, offering a scalable and atom-efficient approach to the functionalization of complex molecules. Our results highlight the broad applicability of this strategy, particularly in the rapid and efficient late-stage diversification of bioactive compounds. The ability to directly use abundant gaseous feedstocks opens new possibilities for sustainable and practical synthetic routes.