That sticky byproduct of smoke and burning wood is called soot. Smoke which escape from fireplace kind of build on the mantel. Just with the water and regular soap soot is not bound to get clear, as this is not enough for cutting the sticky grime of soot which particularly happens on the painted surfaces or on mantels especially which have large degree of carving. For this to clean you will need strong cleaner to get job done.
Here are steps to remove soot from painted mantle.
Put rubber gloves which will protect your skin of hands and also wear eye protection like the transparent goggles.
Now mix together 1/4th cup of the trisodium phosphate cleaner that is TSP with warm water (2 gallons). TSP cleaning agent is available in hardware stores, its form of lye which is sued for cutting through soot as well as grease, as it make them soapy in nature. TSP is used for cleaning mildew, and grease for painted surfaces.
Dip stiff bristled scrub brush in TSP solution.
Apply solution to soot on mantle and then begin scrubbing in circular motion.
For carvings on mantle use small brush to get it clean.
Dip brush frequently into solution while cleaning and cutting through soot and grime.
Take clean sponge and soak in clean water.
Wipe and rinse mantle to remove the last of soot and TSP mixture.
This is strong cleaner so protect your bare skin and eyes.
TSP alternative is sodium carbonate, zeolites, soda ash
Never use TSP or TSP alternative directly always dilute it with the water else it can stain wood and damage mantle.