I am from Ohio and fish the Old Hickory Steam Plant every winter. There is a small ramp just above the plant and one below the plan about 2 miles down river.
To get to the small ramp just above the plant from Gallatin, TN, take 109 across the lake and about 3 miles after crossing the lake turn left on Academy Rd. Follow Academy Rd about 2 and a half miles and make a very hard left onto Coles Ferry Rd. Follow Coles Ferry Rd another 2 miles or so and it will dead end at the ramp.
If you want to use the ramp below the steam plant, just before crossing over the lake on 109, turn left onto Odoms Bend Rd. go about 1/4 mile and the ramp will be on the right at Flippers Bait and Tackle shop. This is a much nicer ramp, but it is a cold 2 mile ride up the lake to the steam plant. Good luck and let me know how you do!