For efficient design and performance of condensers in traditional and modern applications (air-conditioners, looped heat-pipes in space applications, etc.) a need exists— from both fundamental [1] and application [2] perspectives—for tools and techniques that facilitate attainment and accurate predictions of steady annular/stratified internal condensing flows. This paper develops predictive tools for wavy-interface situations involving turbulent vapor/laminar condensate flows in a vertical tube. The flow is quasi-steady—i.e., steady mean flow fields experience wave-induced fluctuations. The flows are sufficiently fast that the entire flow is “parabolic”—i.e., conditions at any point are influenced only by conditions upstream. Situations where the vapor flow is slow and “elliptic” are discussed in the paper [3].