A combined heat and power plant includes a gasifier, a heat exchanger arranged to reduce the temperature of the raw synthesis gas formed in the gasifier by exchanging the heat of the raw synthesis gas into heating medium used for heating and forming cooled raw synthesis gas, a filtration unit for cleaning the cooled raw synthesis gas to form refined synthesis gas suitable as a fuel for an internal combustion engine, an internal combustion engine where the refined synthesis gas is burnt to produce mechanical power, ducts for connecting different parts of the plant to each other a raw gas burner arranged after the gasifier to burn the raw synthesis gas formed in the gasifier during the time when the refined synthesis gas is not utilized in the internal combustion engine. A method for treating raw synthesis gas a combined heat and power plant is also disclosed.