This invention proposes a method of removing pollutants from high temperature gases prior to their introduction in to a heat exchanger of a steam generating boiler within a power generation system. The method places a filter system (100) in a hot gas flow path upstream of a waste-heat boiler. The filter system has at least two banks of substantially hollow elongate filters (110), each of which filters particles from the gas flow as it flows from an external surface thereof to an internal surface thereof. Valves (104, 106) enable each bank of filters to be isolated from the hot gas flow path. The method included periodically isolating at least one bank of filters while passing hot gasses through at least another of the banks of filters, and cleaning the isolated bank of filters. After cleaning the filters (110) lime is injected into the flow path so that it coats the outer surface of the filters (110) to produce a loose lime coating thereon.