Ammonia plays critical role as the second most produced chemical commodity with around 80% used in producing nitrogen-based fertilizer. Considering the decline of reserves of fossil-based feedstocks it is imperative to shift towards greener alternatives. However, such green ammonia processes are far from being economically viable. This makes natural gas-based ammonia synthesis the best available technology currently, but this faces critical difficulties as natural gas supply could widely vary due to declining reserve or changing sources, posing another key challenge to improve the efficiency of affected ammonia production. This study is the first to investigate the effect of variable natural gas composition (within the range of 83–99.99% vol dry methane; towards lean gas) on an industrial ammonia production process with maintained key operating parameters value such as steam to carbon ratio (S/C), hydrogen over nitrogen ratio (H/N), etc. The sensitivity analysis shows that sustained energy efficiency of the process is possible, confirming the conventional ammonia plant's ability to withstand changes in feedstock and fuel supply. In addition, lean gas yielded a positive impact on the raw material intensity and CO₂ emissions with average reductions of 1.17% and 1.79% per each 4% methane content increase, respectively.