Atmospheric icing is extremely hazardous to heavier-than-air aircraft, as it badly disrupts their ability to generate aerodynamic lift (the inherent roughness of accreted ice encourages the airflow to separate from the upper surface of the wing, causing the aircraft to stall at a much lower angle of attack, and a much lower lift coefficient, than it would otherwise) and greatly increases aerodynamic drag (the rough, non-streamlined surface of the ice has much the same effect on the aircraft’s motion through the air as gluing a big sheet of 00-grit sandpaper to your butt would have on your motion down a playground slide). As such, built-in anti-ice equipment (generally consisting of heaters using either engine bleed air or electrical-resistance heating elements, or of pneumatic deicing boots that mechanically dislodge ice) is generally a must for the wing(s), tail surface(s), and propeller(s) (if any) of airplanes intended to fly in icing conditions, in order that any accumulated ice can be melted or broken off before it can significantly affect the aircraft’s aerodynamics. If these systems are inoperative, the aircraft must not be flown in icing conditions until its anti-ice capability is restored, and aircraft lacking these systems are generally forbidden from entering ice altogether.
The Tupolev Tu-204 is an unusual exception to this. Its wing and tail surfaces are (supposedly) so resistant to ice accumulation that it can fly in heavy icing conditions without needing to be equipped with wing or tail anti-ice equipment:
...The wings and tails are relatively resistant to ice build-up, and as such anti-icing systems are not equipped. Among today’s airliners the Tu-204 is the only one which does not require wing anti-icing systems. During the test flight safety has been confirmed without the anti-icing system on the bearing surfaces and the aircraft obtained Russian and European certificates... [Wikipedia, “Tupolev Tu-204”, section 1.1 (“Technology”).]
This seems like a rather suspect claim; atmospherically-generated ice has the well-documented tendency to stick to essentially whatever it hits (as long as it’s not warm enough to melt the ice off), no matter what its composition, shape, or surface texture. Even if the Tu-204 goes the usual route of making the tail bigger so it works fine even when iced, this doesn't explain how they can get away with non-anti-iced wings; how does the Tu-204 supposedly manage to do this?