A gas cleaning system is operative for removing water vapour from a carbon dioxide rich flue gas generated in a boiler combusting a fuel in the presence of a gas containing oxygen gas. The gas cleaning system comprises a flue gas condenser (12) being operative for condensing water from at least a portion of the carbon dioxide rich flue gas generated in the boiler by bringing the carbon dioxide rich flue gas into contact with a circulating cooling liquid, thereby generating a cleaned carbon dioxide rich flue gas having a lower concentration of water vapour than the carbon dioxide rich flue gas leaving the boiler. The gas cleaning system comprises a heat pump (100) being operative for absorbing heat at a first temperature from the cooling liquid, and for desorbing heat at a second temperature, being higher than the first temperature, to a heat sink (124).