Through consideration of the requirements for a DEMO-relevant blanket concept, Korea (KO) has proposed a He-Cooled Molten Lithium (HCML) Test Blanket Module (TBM) for testing in the International Thermonuclear Experimental Reactor (ITER). To validate the safety of the HCML TBM design concept and guarantee high efficiency of the power conversion system, an evaluation of the heat transfer capability of the gas coolant in a high Reynolds number regime should precede this test. In this study, a thermal hydraulic test with a high-pressure nitrogen gas loop was performed and a thermal hydraulic analysis was carried out with the commercial CFD code Fluent 6.3.26 and the system code GAMMA (Gas Multicomponent Mixture Analysis) under the same test conditions. In the experiment, a single TBM First Wall (FW) mock-up made from the same material as the KO TBM, ferritic martensitic steel, was used, and the test was performed at pressures of 11, 19 and 29 bar and under various flow rates ranging from 0.63 to 2.44 kg/min. As one-side of the mock-up was heated by a furnace heater at a constant temperature, the wall temperatures were measured by installed thermocouples, with the measured temperatures showing strong parity with code results simulated under the same test conditions. Even with the system code using the modified Dittus-Boelter correlation, which was developed under a different heating condition, the three-dimensional approach of the system code is capable of estimating a one-sided heating condition in a fusion application.