A new method of image subtraction is applied to images from a J, K, and narrow-band imaging survey of 300 deg2 of the plane of the Galaxy, searching for new Wolf-Rayet (WR) stars. Our survey spans 150{deg} in Galactic longitude and reaches b=+/-1{deg} with respect to the Galactic plane. The survey has a useful limiting magnitude of K=15 over most of the observed Galactic plane, and K=14 (due to severe crowding) within a few degrees of the Galactic Centre. The new image subtraction method described here (better than aperture or even point-spread-function photometry in very crowded fields) detected several thousand emission-line candidates. In 2011 and 2012 June and July, we spectroscopically followed up on 333 candidates with MDM-TIFKAM and Infrared Telescope Facility (IRTF)-SpeX, discovering 89 emission-line sources. These include 49 WR stars, 43 of them previously unidentified, including the most distant known Galactic WR stars, more than doubling the number on the far side of the Milky Way. We also demonstrate our survey's ability to detect very faint planetary nebulae and other NIR emission objects.