The combined use of coal and biomass in co-gasification plants is an alternative to crude oil in the production of secondary energy carriers such as methane and methanol. Despite the higher costs by the use of biomass, an incentive could be savings on the CO2 certificates. Thus, this work aims to determine the certificate price leading to a competitive production of methane and methanol by co-gasification processes. Therefore, the greenhouse gas emissions and production costs are calculated and greenhouse gas mitigation costs are determined.