Reflux condensation in the U-tubes of a steam generator (SG) plays a vital role in removing heat in pressurized water reactors (PWRs) during mid-loop operations and/or accidents. To evaluate the efficacy of reflux condensation in SG U-tubes, a comprehensive understanding of the heat transfer characteristics associated with pure steam is necessary. Therefore, this study performs experiments to examine the reflux condensation of pure steam within a vertical tube with an inner diameter of 19 mm and an outer diameter of 22 mm. The steam bulk temperatures were systematically altered at 10-degree intervals within the range of 120–150 °C, and the steam Reynolds number ranged from 4876 to 20,430. Throughout, the temperature of the coolant within the cooling jacket and its rate of flow remained constant at 55 °C and 8 L/min, respectively. Validation via the TRACE system code confirmed good agreement with the experimental results, further bolstering confidence in the consistency of the current findings with the data previously used to calibrate and refine the correlations within the system code.