A number of factors must be considered in the design of miscible displacement processes. This paper discussus a new approach that relates primarily to the laboratory and modelling studies that precede the EOS-based compositional sirmilation of reservoir performance during the vapourizing/condensing gas drive process. The phase behaviour of a solvent/oil system and determination of miscibility conditions by various special PVT experiments (including the swelling test, RBA, slim tube test, and the continuous I multiple-contact experiment) are reviewed along with their importance in building an accurate EOS model to be used in compositional simulation. In addition to experimental PVT data, a special core flow test design for measuring the relative permeabilities to generated fluids by forward/reverse multiple contact experiments is discussed. Based on laboratory PVT and SCAL data, a novel interfacial tension-dependent model of relative permeability and capillary pressure data is presented along with the advantages if incorporated into commercial EOS-based compositional simulation software packages.