Syngas biomethanation is an emerging technology that converts synthesis gas, primarily composed of hydrogen (H<inf>2</inf>), carbon monoxide (CO), and carbon dioxide (CO<inf>2</inf>), into methane (CH<inf>4</inf>) through microbial activity. In this study, the effect of changing syngas composition with increased H<inf>2</inf> shares on CH<inf>4</inf> concentration and production was assessed for 125 days, using a thermophilic trickle-bed reactor (5 L). With the experimental upper limit of 71 % H<inf>2</inf> (14 % CO, 10 % CO<inf>2</inf>, 5 % N<inf>2</inf>) in the syngas, the maximum CH<inf>4</inf> concentration was 65 %, maintaining high methane evolution rates (4 L/(L<inf>pbv</inf>·d)) and high H<inf>2</inf> and CO conversion rates (>95 %). Targeted sulfur supplementation (Na<inf>2</inf>S) did not improve H<inf>2</inf> and CO conversion or CH<inf>4</inf> productivity, indicating that sulfur was no limiting factor under digestate-based operation. Reactor performance was instead constrained by system-level factors, including low gas retention time, gas–liquid mass transfer limitations, and inhibition of CO-converting pathways at elevated H<inf>2</inf> partial pressure. 16S rRNA gene sequencing revealed a highly stable microbial community dominated by the hydrogenotrophic methanogen Methanothermobacter . CO conversion occurred via direct methanogenesis and acetate formation, followed by syntrophic acetate oxidation. Overall, increasing H<inf>2</inf> availability enhanced CH<inf>4</inf> production only up to a system-specific threshold, beyond which microbial and transport limitations dominated