High temperature heat pumps can be used to generate high temperature air for various industrial applications, such as drying process. Transcritical CO2 heat pumps have gained popularities due to utilizing low global warming potential refrigerants. In order to heat up the air, a fin-and-tube gas cooler heat exchanger, where supercritical CO2 (s-CO2) flows inside the tubes, is considered. To reduce the pressure-drop and increase the heat transfer area, large inner diameter tubes, e.g., inner diameter (ID) = 22.14 mm, are usually selected. Due to significant density variations and its substantial dependency on spatial pressure and temperature of s-CO2, specially near pseudo-critical operating condition, buoyancy effect significantly influences the heat transfer and flow hydrodynamic of s-CO2 in the tubes. In this study, 3D computational fluid dynamic simulations have been performed to evaluate the heat transfer and pressure drop in two horizontal tubes connected with one U-bend. All governing equations are solved with a commercial finite volume solver. The k-ω SST model with near wall low-Reynolds number correction is used to include the turbulence and strong buoyance effects near wall. The accuracy of the obtained results is validated against experimental data for heat transfer in horizontal tube, showing good agreement. Simulations are performed for three practical s-CO2 mass flowrate of 0.05, 0.1, and 0.15 kg s - 1 in both upwards and downwards s-CO2 flow in the U-bend at operating pressure of 8 MPa, with an inlet temperature 0.5K above the corresponding pseudo-critical temperature. The numerical results indicate that both heat transfer rate and pressure drop increases in the upward flow in the U-bend compared to the equivalent straight tube by 7.42% and 40.8%, respectively.