Yes, chickens are definitely susceptible to heat stress and can die when temperatures get high. Chickens are most comfortable in moderate temperatures. They cannot sweat - they cool themselves by panting. 

You can help keep them cool by providing extra shade to get out of the sun with tarps or other shelters. Make sure your shaded areas have proper ventilation. You can try adding a fan to get air moving. Always provide lots of fresh, clean, cool water. If you normally use nipple drinkers, offer water in a large open pan or deep cups. Try adding ice cubes or frozen water bottles to keep water temperature down. Minimize other stressors (keep the dog in, keep your roosters apart, don't let the kids chase the chickens, etc). 

Watch for signs of heat exhaustion which occurs if the bird's core body temperature gets too high. Pay particular attention to larger birds, older birds, dark colored birds, and birds with pea combs. They may have more trouble dealing with the heat.

Signs of Heat Exhaustion
-heavy panting
-holding wings away from body
-lethargic
-pale or discolored comb and wattles
-unconscious

If your chicken is showing the signs of heat exhaustion above, you need to work quickly to cool them down. Get them out of the sun and into a cool place (such as the house). Bring their body temperature down by submerging their body in a bucket of cool (but not icy) water. Towel dry and keep them in a cool, dimly lit area to keep them calm and provide cool water with electrolytes.

While the heat can be dangerous for you and your chickens, many birds make it through hot summers just fine with a little help.