Engine idles OK and normal, but when accelerate, sounds muffled and has no power. Thought I may have a head gasket problem due to seeing some white smoke. Exhaust smells foul.

Sounds like a head gasket leaking water in from the cooling system. Does it run OK? Any water in the oil on the dipstick and a mayonnaise type film on the oil filler cap?

Head gasket leaking?

The white smoke suggests head gasket failure.

If the dealer is calling a head gasket i would trust them. Water on the dipstick isnt something you would see. chances are coolant is getting into the cylinders and burning as you drive so you are probably low on coolant and theres enough to flow when the engine rpm is high. Which would allow some heat when driving and none when idling. Also if there was water in the engine, it would sit at the bottom due to oil being lighter than water so if anything it would just look like you ave to much oil on the dipstick. The steam your seeing might actually be exhaust fumes from a blown head gasket

Could be a clogged catalytic converter. It could also be a bad head gasket allowing some coolant to enter the cylinders. Check for milky grey oil.

White smoke and running roughly would normally point to a head gasket problem.

Keep an eye on your engine coolant I'd imagine it will start to disappear.

Even though coolant level is OK, it does not rule out a blown head gasket-has the engine recently been overheated? You should get engine compression checked, to see if you have low compression in 2 adjacent cylinders, to rule out the possibility. White smoke and very rough engine performance point to a blown head gasket, however.

the head gaskets are blown you can believe that anytime on any engine gas or deisel that you get oil in the radiator and there water/ antifreeze in the oil pan the gaskets and or cylinder heads are bad

it sounds like the engine coolant is leaking into the combustion chambers most likely due to a leaky head gasket or a cracked cylinder head and will cause loose of power in acceleration

This is usually an indication of a bad head gasket.I gasket. The white smoke is the coolant leaking out of the engine and winding up in the oil passages, due to a bad head gasket. To confirm this,
1. check oil. Is level increased, or milky appearance?
2. Is coolant level dropping, but u don't see any signs of leaks?
The above indicates a bad head gasket, causing the white smoke u describe. Let me know how u make out.