When your specific objective is to ONLY gain lean muscle and not to gain any fat at the same time then you need to train with a high intensity. There are countless variations on which training routine will gain muscle so in order to focus on only gaining lean muscle we need to look at your daily nutrition that you are getting.
The first thing that you are going to need if you want to gain lean muscle is to create a carbohydrate strategy that you can implement. Timing is everything and your quality complex carbohydrates should be eaten first thing in the morning and within 90 minutes after your workout.
The second is to make sure that when you reduce your carbs that you also increase the fats that you take in. Tim McComsey who is a trainer, dietician as well as the chief correspondent to the HUMANFITPROJECT says "Fat doesn't make you fat, too much food makes you fat."
The third tip to gain lean body mass is to replace your normal fruits that you would eat during the day with veggies. Fruit is full of anti-oxidants and necessary nutrients but you also get them from veggies without the extra carbs added.
The fourth tip is to stop doing any cardio and rather do circuit training with weights, however the intensity of your workouts can be easily adjusted by you and the time that you rest between sets. You can get all the cardiovascular benefits by increasing intensity.
The last point that needs to be seriously considered if your objective is to gain any lean muscle to stop drinking alcohol. Tim McComsey says "Alcohol causes fat to be stored instead of being used for energy. Essentially, your body will put a hold on the digestion of any food until the liver can excrete the alcohol."
The above are just a few tips that will help guide you to gaining good quality lean muscle over a period of time. There are more and more top professional bodybuilders who only use this method of gaining muscle because if your body-fat is constant any weight that you gain is pure lean muscle tissue.