Many of the synths and sound modules that have found favour in home recording produce low‑level outputs designed to interface with hi‑fi equipment rather than serious mixing consoles. The result is that you have to turn the mixer's line gain right up to get enough level, which also tends to increase the level of background noise. A more satisfactory solution is to feed the output from the synthesizer into a console mic input (or pair of inputs in the case of a stereo instrument) which makes much more gain available.
This is fairly straightforward, but there are a couple of points to watch, the most important being that the phantom power is not applied to the socket in question. If the phantom power is switched in, there's a chance that the synth's output stage will be damaged.
You should also ensure that the lead you use is wired correctly. Most mic inputs are on balanced XLRs, so make up a cable with an unbalanced jack at one end and an XLR plug at the other. The hot signal from the tip of the jack should be connected to pin 2 of the XLR plug, while the screen should be connected to both pins 1 and 3 of the XLR.
Very occasionally you'll come across a synth module that has both a low level output and a higher than normal output impedance. In this case, the mic input of the mixer will present too low a load impedance and may compromise the sound. The best option in this case is to use an active DI box with a high input impedance.