This study presents the concept of the falling-film thermosyphon heat exchanger for harvesting the water vapor present in the flue gas of a natural gas combined-cycle (NGCC) power plant. Water vapor is condensed on the outer shell of the polymer tubes in the evaporator section of the thermosyphon, cooled by the evaporation of the falling film inside the tubes. Liquid water is actively pumped to the top of the evaporator to exchange heat in axial co-flow with the flue gas outside the tubes. A lab-scale thermosyphon system has been built to run the condensation test to validate this concept. The heat and mass transfer was analytically modeled by ε-NTU method. This paper shows the results and compares the model to the experiment.