PROBLEM TO BE SOLVED: To provide a two-stage system to cause flow of flue gas and treat the gas only in a downflow direction by a method wherein most of the sensible heat of flue gas is removed in a first heat-exchanger, its latent heat is removed in a second heat-exchanger therebelow, and liquid drops produced due to condensation in the two heat-exchangers drop downward. SOLUTION: Condensation type first and second heat-exchangers 56 and 62 are mounted in a state to be arranged in series in a vertical direction. Flue gas 52 enters from the apex of a device 50 through an inlet 51 and is discharged through an outlet 68 through the bottom of the device 50. After most of the sensible heat of the flue gas 52 is removed in the first heat-exchanger 56, the gas flows through a migration section 60, enters the second heat-exchanger 62 located therebelow, and a latent heat is removed at the second heat- exchanger 62. Liquid drops are generated in the first and second heat-exchangers due to condensation and drop downward by an influence exercised by downward movement of gravity and a flow of the flue gas 52.
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