Drop coalescence and ink reflow significantly affect the image quality in inkjet printing, especially on coated and non-absorbent substrates. In this study, the coalescence of inkjet-printed drops on a non-absorbent substrate was numerically simulated and compared with machine tests. Using a commercial software Flow-3D, the coalescing drop pairs are simulated by solving the mass and momentum conservation equations with Volume-of-Fluid method tracking the liquid-air interface. The effects of drop spacing and time interval on the geometry of coalescing drops are numerically investigated. Results show that when the second drop lands next to a pre-deposited drop, liquid in the second drop migrates towards the first drop driven by its internal capillary pressure and inertia. As a result, a geometrical shift towards the first drop and reduced drop spreading length are observed in the merged drop. The geometric shifts of coalesced droplets were qualitatively proved in a machine test using statistical image analysis. Both the reduced drop spreading length and the geometric shift contribute to the trailing edge displacement, defined as the wetting edge shift of the second drop away from its ideal location. It is also indicated in the simulation that drop spacing and time interval influence the geometric shift, drop spreading length, and thus trailing edge displacement. This trailing edge displacement plays an important role in determining the continuity of the printed lines, e.g. noncoalescence of printed drops due to large trailing edge displacement will lead to broken patterns or functional failures of printed electronics. This study provides valuable information in designing jetting sequence in the printhead, and optimizing printing processes for uniform line and solid formation.