Don't buy anything HARTZ, not even their stupid plastic chew toys from china. They are evil murdering money-sucking scumbags.
Your dog is suffering from neurological toxins/depressants that were supposed to kill the fleas. They are numbing his nervous system making him very sick. Wash dog with soap/shampoo along spine a couple of times to try to keep any more of it from being absorbed.
Tell your vet your dog had a bad reaction to Hartz and ask what he recommends.
Our dog went into severe shock as well and it f*cked him up for a long time and made him hypersensitive to alot of nerve delivered flea treatments, also I think his wonky gacking is a result of nerve damage from HARTZ. Consider keeping your dog in his pen, and avoid trying to force him to do anything physical. If it is really bad cover his eyes and see if that helps. Our dog could only lean against walls and lay on the floor because the room was spinning so much for him.
We use Advantage for fleas now with no problems. Advantix is the version for flea and ticks, but it is too strong for him. It is pretty easy to get into a daily routine for checking for ticks by hand so we just use a flea treatment.
Good luck, and research any product you are going to give to your dog. Like you, we learned the hard way.