Science-Based Emissions Standards Act of 2021 This bill prohibits the Environmental Protection Agency (EPA) from finalizing any rule to amend the new source performance standards for stationary sources under the Clean Air Act to reduce methane emissions from new and modified oil and natural gas facilities until the Department of Energy takes specified actions. Specifically, the Department of Energy must first (1) complete and publish the results of the study titled Quantification of Methane Emissions from Marginal (Small Producing) Oil and Gas Wells, and (2) provide a period of at least 90 days for public comment on the application of the study's results. The EPA must take the results of the study into consideration when finalizing the rule.