The invention discloses a bypass flow horizontally-arranged gap-type condensation heat exchanger. A multi-layer coil tube structure is adopted; the main component of the heat exchanger is a heat exchanger coil tube which is formed by rolling a rectangular tube along a long side direction, and the short side of the heat exchanger coil tube is located in the thickness direction of the heat exchanger coil tube; two ends of the heat exchanger coil tube are provided with an inner water collecting tube and an outer water collecting tube; heated working medium water uniformly flows from the inner water collecting tube to each coil tube, and rotates in the coil tubes from inside to outside for multiple circles, and then enters into the outer water collecting tube; high-temperature smoke generated after combustion in a combustor flows from inside to outside along gaps among the coil tubes; the flow of the smoke in the gaps is laminar flow; because the dimensions of the gaps are very small, the boundary layers are very thin, the temperature gradient is extremely great, and the smoke passing through the gaps can obtain large heat exchange coefficient; and between the smoke and the heated working medium water are in a converse distributary heat exchange mode, thus the average heat exchange temperature difference is improved, the smoke exhaust temperature is lowered effectively, steam in the smoke is condensed as much as possible, and much condensation heat is absorbed and utilized.