To maximize a heat recovery amount without generating low-temperature corrosion in a heat exchanger, even when properties of an exhaust gas rapidly or slowly changes due to change of wastes and the like.

In this method of operating a waste incineration plant including a waste incinerator 1, a boiler 2 and an economizer 3 for recovering heat of the exhaust gas G discharged from the incinerator 1, and an exhaust gas processing system for removing soot and dust, an acidic gas and nitrogen oxide in the exhaust gas G passing through the economizer 3, an acid dew-point and SO2 concentration in the exhaust gas G at an inlet side of the economizer 3 are continuously measured, urea or ammonia is sprayed into the exhaust gas G in the incinerator 1 and at the inlet side of the economizer 3 on the basis of a result of the measurement of the acid dew point to remove SO3 so that the acid dew point temperature in the exhaust gas G is lowered, and a temperature of supply water W supplied to the economizer 3 and a boiler 2 is controlled on the basis of a result of the measurement of SO2 concentration to keep a surface temperature of a heat transfer tube of the economizer 3 at the acid dew point temperature or more.