The large amount of greenhouse gases (GHG) emitted by anthropogenic action becomes a challenge that requires initiatives for their mitigation, such as public-private partnerships. In developing countries, this issue is even more problematic, as growing economic development is not accompanied by legislation and initiatives regulating methane released into the atmosphere. This paper aims to measure GHG reduction by applying a United Nations Clean Development Mechanism (CDM) design methodology to a real landfill. The methods used were a case study of a landfill in the state of Rio de Janeiro, Brazil, applying the United States Environmental Protection Agency's LandGem model for the sizing of the produced gases and, finally, the CDM project analysis to define a methodology that applies to the case. The results show that the simple implementation of landfill flares greatly reduces methane emissions, but greater public and private interest in the application of gas mitigation methodologies is still needed. The main limitation of this research is the fact that there are few publications in the area of CDM project methodologies, and that the data used to size the gases in the LandGem model were estimated through population and waste forecasts. Finally, this study can be widely applied in landfills of different locations, not restricted to Brazil.