We present results from SEPPCoN, our Survey of Ensemble Physical Properties of Cometary Nuclei. This survey involves studying 100 Jupiter-family comets (JFCs) -- about 30% of the known population -- at both mid-infrared and visible wavelengths. We have used the Spitzer Space Telescope to study the comets' thermal emission, and many ground-based telescopes (Apache Point's ARC 3.5-m; ESO's NTT 3.6-m; MKO's UH 2.2-m and Keck 10-m; Palomar's 5-m and 1.5-m; ORM's WHT 4.2-m, NOT 2.6-m, and LT 2-m; Cerro Pachon's SOAR 4.1-m) to study the reflected sunlight. The Spitzer observations (imaging with IRS PU and MIPS) are complete, and the ground-based observations (imaging in at least R band) are about half complete. Almost all our targets are imaged while farther than 4 AU from the Sun, to minimize (and often eliminate) confusion caused by dust from cometary activity. The Spitzer data constrain the effective radii of the nuclei; we find preliminarily that the cumulative size distribution's power-law slope is similar to what has been found by others using visible wavelength studies, suggesting that there is no strong trend of albedo with size. The Spitzer data also tell us about the thermal inertia, and we find that many -- though not all -- cometary nuclei seem to have low values of this, consistent with a porous, fluffy, poorly-conducting, dusty surface layer. The Spitzer images show that about one-third of our sample appeared with extended dust emission despite being close to aphelion. We find that often the activity observed at these distances seems to turn off abruptly once a JFC passes aphelion. We have used dynamical analysis to constrain the dust grain sizes and thereby distinguish dust tails from dust trails. The dust temperatures are in most cases consistent with isothermal, low-albedo grains in LTE. We thank the Spitzer Science Center and the TACs of the aforementioned telescopes for supporting this research.