A correlation is presented for the interfacial friction factor between the gaseous and liquid phases in vertical two-phase flows past horizontal in-line and staggered tube bundles. The interfacial friction data were determined from pressure drop, void fraction, and mass flux data taken by Dowlati et al. (1990, 1992b) and Schrage et al. (1988). These data were correlated using two non-dimensional quantities : a Reynolds number based on the mixture density and relative velocity between the two phases, and the porosity of the tube bundle. These dependencies are explained in terms of the competing effects of viscosity and buoyancy and the influence of turbulence in the flow. The correlation is implemented in a numerical simulation of the recirculating shell-side flow encountered in kettle reboiler heat exchangers, the results of which are compared to the experimental observations of Cornwell et al. (1980).